Automotive internal combustion engines occasionally receive a momentarily overly-rich air-fuel mixture upon movement of the engine throttle from an open position, perhaps due to evaporation of liquid fuel in the induction system. The rich mixture may lead to a brief pulse of engine power, to increased emission of unburned hydrocarbons and carbon monoxide, or to exhaust system backfire noise. Thus in such an engine, a valve may be employed to bleed additional air into the induction system to lean the air-fuel mixture as the throttle is moved from an open position, thereby preventing the brief pulse of engine power, minimizing the emission of exhaust gas hydrocarbons and carbon monoxide, and avoiding exhaust system backfire noise.
The previous valve cannot be employed in a turbocharged engine, however, for that valve responded to a rapid change in induction passage pressure which does not occur in a turbocharged engine.